The present invention relates to a variable bitrate video coding method including an iterative process that comprises a first analysis step, for coding a bitstream corresponding to a picture sequence with a constant quantization stepsize, and a second prediction step, for predicting the quantization stepsize which must then be used to code said bitstream according to a predetermined target bit rate, and is followed by a final control step, for adjusting the stepsize with respect to said target bit rate. The invention also relates to a corresponding video coder allowing to carry out said method.
As described in the document &lt;&lt;MPEG Video coding: a basic tutorial introduction &gt;&gt;, S. R. Ely, BBC Research and Development Report, BBC-RD-1996/3, pp. 1-10, MPEG activities started in 1988 with the aim of defining standards for digital compression of video and audio signals. The first goal was to define a video algorithm for digital storage media such as the CD-ROM (Compact Disc Read-Only Memory), but the resulting standard was also applied in the Interactive CD system (CD-I). Allowing transmission and storage of picture data at bit rates in the range of 1 to 15 Mbits/s, this standard is based on a data compression achieved by using a block-based motion compensation for the reduction of the temporal redundancy and a discrete cosine transformation (DCT) for the reduction of the spatial redundancy.
With conventional CD standards such as CD-I and CD-ROM, the transmission bit rate is fixed and pictures can therefore only be coded at a constant bit rate. New standards such as the Digital Versatile Disc (DVD) allow for transmission of data at a variable bit rate (VBR): complex scenes can be coded at a higher bitrate than scenes containing less information, in order to maintain a constant quality.
An object of the invention is to propose a VBR video coding method allowing to obtain such a constant quality of the coded sequence, with a minimal bit rate of the encoder output bitstream.